Try and contact them by e-mail when you're in London perhaps asking for them to rebook you onto the next flight -as a partner of BA they may be willing:

[email protected]

The flights tomorrow have good availability:

1220 1345 PG 924 Y7 M7 K7 N7 T0 L0 H0
1355 1520 PG 273 Y7 M7 K7 N7 T7 L7 H3
1655 1820 PG 277 Y7 M7 K7 N7 T7 L7 H6
1945 2110 PG 279 Y7 M7 K7 N7 T7 L7 H6 Q1 V1
2155 2320 PG 220 Y7 M7 K7 N7 T7 L7 H7 Q7 V5

If on arrival they won't help you and want you to pay for another ticket, check with Thai who have far more flights to HKT as well.